Difference between revisions of "Android Started Service – A Kotlin Example"

Latest revision as of 16:29, 4 December 2017

The previous chapter covered a considerable amount of information relating to Android services and, at this point, the concept of services may seem somewhat overwhelming. In order to reinforce the information in the previous chapter, this chapter will work through an Android Studio tutorial intended to gradually introduce the concepts of started service implementation.

Within this chapter, a sample application will be created and used as the basis for implementing an Android service. In the first instance, the service will be created using the IntentService class. This example will subsequently be extended to demonstrate the use of the Service class. Finally, the steps involved in performing tasks within a separate thread when using the Service class will be implemented. Having covered started services in this chapter, the next chapter, entitled “Android Local Bound Services – A Worked Example”, will focus on the implementation of bound services and client-service communication.

Creating the Example Project

Launch Android Studio and follow the usual steps to create a new project, entering ServiceExample into the Application name field and ebookfrenzy.com as the Company Domain setting before clicking on the Next button.

On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 14: Android 4.0 (IceCreamSandwich). Continue to proceed through the screens, requesting the creation of an Empty Activity named ServiceExampleActivity using the default values for the remaining options.

Creating the Service Class

Before writing any code, the first step is to add a new class to the project to contain the service. The first type of service to be demonstrated in this tutorial is to be based on the IntentService class. As outlined in the preceding chapter (“An Overview of Android Started and Bound Services”), the purpose of the IntentService class is to provide the developer with a convenient mechanism for creating services that perform tasks asynchronously within a separate thread from the calling application.

Add a new class to the project by right-clicking on the com.ebookfrenzy.serviceexample package name located under app -> java in the Project tool window and selecting the New -> Kotlin File/Class menu option. Within the resulting Create New Class dialog, name the new class MyIntentService and select Class from the Kind menu. Finally, click on the OK button to create the new class.

Review the new MyIntentService.kt file in the Android Studio editor where it should read as follows:

package com.ebookfrenzy.serviceexample
 
/**
 * Created by <named> on <date>.
 */
class MyIntentService {
}

The class needs to be modified so that it subclasses the IntentService class. When subclassing the IntentService class, there are two rules that must be followed. First, a constructor for the class must be implemented which calls the superclass constructor, passing through the class name of the service. Second, the class must override the onHandleIntent() method. Modify the code in the MyIntentService.kt file, therefore, so that it reads as follows:

package com.ebookfrenzy.serviceexample
 
import android.app.IntentService
import android.content.Intent
 
class MyIntentService : IntentService("MyIntentService") {
 
    override fun onHandleIntent(arg0: Intent?) {
 
    }
}

All that remains at this point is to implement some code within the onHandleIntent() method so that the service actually does something when invoked. Ordinarily this would involve performing a task that takes some time to complete such as downloading a large file or playing audio. For the purposes of this example, however, the handler will simply output a message to the Android Studio Logcat panel:

package com.ebookfrenzy.serviceexample
 
import android.app.IntentService
import android.content.Intent
import android.util.Log
 
class MyIntentService : IntentService("MyIntentService") {
 
    private val TAG = "ServiceExample"
    
    override fun onHandleIntent(arg0: Intent?) {
        Log.i(TAG, "Intent Service started")
    }
}

Adding the Service to the Manifest File

Before a service can be invoked, it must first be added to the manifest file of the application to which it belongs. At a minimum, this involves adding a <service> element together with the class name of the service.

Double-click on the AndroidManifest.xml file (app -> manifests) for the current project to load it into the editor and modify the XML to add the service element as shown in the following listing:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ebookfrenzy.serviceexample">
 
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".ServiceExampleActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
 
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <service android:name=".MyIntentService" />
    </application>
 
</manifest>

Starting the Service

Now that the service has been implemented and declared in the manifest file, the next step is to add code to start the service when the application launches. As is typically the case, the ideal location for such code is the onCreate() callback method of the activity class (which, in this case, can be found in the ServiceExampleActivity.kt file). Locate and load this file into the editor and modify the onCreate() method to add the code to start the service:

package com.ebookfrenzy.serviceexample
 
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.content.Intent
 
class ServiceExampleActivity : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_service_example)
        
        val intent = Intent(this, MyIntentService::class.java)
        startService(intent)
    }
}

All that the added code needs to do is to create a new Intent object primed with the class name of the service to start and then use it as an argument to the startService() method.

Testing the IntentService Example

The example IntentService based service is now complete and ready to be tested. Since the message displayed by the service will appear in the Logcat panel, it is important that this is configured in the Android Studio environment.

Begin by displaying the Logcat tool window before clicking on the menu in the upper right-hand corner of the panel (which will probably currently read Show only selected application). From this menu, select the Edit Filter Configuration menu option.

In the Create New Logcat Filter dialog name the filter ServiceExample and, in the by Log Tag field, enter the TAG value declared in ServiceExampleActivity.kt (in the above code example this was ServiceExample).

When the changes are complete, click on the OK button to create the filter and dismiss the dialog. The newly created filter should now be selected in the Android tool window.

With the filter configured, run the application on a physical device or AVD emulator session and note that the “Intent Service Started” message appears in the Logcat panel. Note that it may be necessary to change the filter menu setting back to ServiceExample after the application has launched:

06-29 09:05:16.887 3389-3948/com.ebookfrenzy.serviceexample I/ServiceExample: Intent Service started

Had the service been tasked with a long-term activity, the service would have continued to run in the background in a separate thread until the task was completed, allowing the application to continue functioning and responding to the user. Since all our service did was log a message, it will have simply stopped upon completion.

Using the Service Class

While the IntentService class allows a service to be implemented with minimal coding, there are situations where the flexibility and synchronous nature of the Service class will be required. As will become evident in this chapter, this involves some additional programming work to implement.

In order to avoid introducing too many concepts at once, and as a demonstration of the risks inherent in performing time-consuming service tasks in the same thread as the calling application, the example service created here will not run the service task within a new thread, instead relying on the main thread of the application. Creation and management of a new thread within a service will be covered in the next phase of the tutorial.

Creating the New Service

For the purposes of this example, a new class will be added to the project that will subclass from the Service class. Right-click, therefore, on the package name listed under app -> java in the Project tool window and select the New -> Service -> Service menu option. Create a new class named MyService with both the Exported and Enabled options selected.

The minimal requirement in order to create an operational service is to implement the onStartCommand() callback method which will be called when the service is starting up. In addition, the onBind() method must return a null value to indicate to the Android system that this is not a bound service. For the purposes of this example, the onStartCommand() method will loop 3 times sleeping for 10 seconds on each loop iteration. For the sake of completeness, stub versions of the onCreate() and onDestroy() methods will also be implemented in the new MyService.kt file as follows:

package com.ebookfrenzy.serviceexample
 
import android.app.Service
import android.content.Intent
import android.os.IBinder
import android.util.Log
 
class MyService : Service() {
 
    private val TAG = "ServiceExample"
 

    override fun onCreate() {
        Log.i(TAG, "Service onCreate")
    }
 
    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
 
        Log.i(TAG, "Service onStartCommand " + startId)
 
        var i: Int = 0
 
        while (i <= 3) {
 
            try {
                Thread.sleep(10000)
                i++
            } catch (e: Exception) {
            }
            Log.i(TAG, "Service running")
        }
        return Service.START_STICKY
    }
 
    override fun onBind(intent: Intent): IBinder? {
        Log.i(TAG, "Service onBind")
        return null
    }
 
    override fun onDestroy() {
        Log.i(TAG, "Service onDestroy")
    }
}

With the service implemented, load the AndroidManifest.xml file into the editor and verify that Android Studio has added an appropriate entry for the new service which should read as follows:

<service
 android:name=".MyService"
            android:enabled="true"
            android:exported="true" >
</service>

Modifying the User Interface

As will become evident when the application runs, failing to create a new thread for the service to perform tasks creates a serious usability problem. In order to be able to appreciate fully the magnitude of this issue, it is going to be necessary to add a Button view to the user interface of the ServiceExampleActivity activity and configure it to call a method when “clicked” by the user.

Locate and load the activity_service_example.xml file in the Project tool window (app -> res -> layout -> activity_service_example.xml). Delete the TextView and add a Button view to the layout. Select the new button, change the text to read “Start Service” and extract the string to a resource named start_service.

With the new Button still selected, locate the onClick property in the Attributes panel and assign to it a method named buttonClick.

Next, edit the ServiceExampleActivity.kt file to add the buttonClick() method and remove the code from the onCreate() method that was previously added to launch the MyIntentService service:

package com.ebookfrenzy.serviceexample
 
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.content.Intent
import android.view.View
 
class ServiceExampleActivity : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_service_example)
 
    }
 
    fun buttonClick(view: View)
    {
        intent = Intent(this, MyService::class.java)
        startService(intent)
    }
}

All that the buttonClick() method does is create an intent object for the new service and then start it running.

Running the Application

Run the application and, once loaded, touch the Start Service button. Within the Logcat tool window (using the ServiceExample filter created previously) the log messages will appear indicating that the onCreate() method was called and that the loop in the onStartCommand() method is executing.

Before the final loop message appears, attempt to touch the Start Service button a second time. Note that the button is unresponsive. After approximately 20 seconds, the system may display a warning dialog containing the message “ServiceExample isn’t responding”. The reason for this is that the main thread of the application is currently being held up by the service while it performs the looping task. Not only does this prevent the application from responding to the user, but also to the system, which eventually assumes that the application has locked up in some way.

Clearly, the code for the service needs to be modified to perform tasks in a separate thread from the main thread.

Creating an AsyncTask for Service Tasks

As outlined in “A Basic Overview of Threads and AsyncTasks”, when an Android application is first started, the runtime system creates a single thread in which all application components will run by default. This thread is generally referred to as the main thread. The primary role of the main thread is to handle the user interface in terms of event handling and interaction with views in the user interface. Any additional components that are started within the application will, by default, also run on the main thread.

As demonstrated in the previous section, any component that undertakes a time consuming operation on the main thread will cause the application to become unresponsive until that task is complete. It is not surprising, therefore, that Android provides an API that allows applications to create and use additional threads. Any tasks performed in a separate thread from the main thread are essentially performed in the background. Such threads are typically referred to as background or worker threads.

A very simple solution to this problem involves performing the service task within an AsyncTask instance. To add this support to the app, modify the MyService.kt file create an AsyncTask subclass containing the timer code from the onStartCommand() method:

.
.
import android.os.AsyncTask
.
.
    private inner class SrvTask : AsyncTask<Int, Int, String>() {
 
        override fun onPreExecute() {
 
        }
 
        override fun doInBackground(vararg params: Int?): String {
 
            val startId = params[0]
 
            var i = 0
            while (i <= 20) {
                try {
                    Thread.sleep(10000)
                    publishProgress(startId)
                    i++
                }
                catch (e: Exception) {
                    return(e.localizedMessage)
                }
            }
            return "Service complete $startId"
        }
 
        override fun onProgressUpdate(vararg values: Int?) {
            super.onProgressUpdate(*values)
            val counter = values.get(0)
            Log.i(TAG, "Service Running $counter")
        }
 
        override fun onPostExecute(result: String) {
            Log.i(TAG, result)
        }
    }
}

Next, modify the onStartCommand() method to execute the task in the background, this time using the thread pool executor to allow multiple instances of the task to run in parallel:

override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
    val task = SrvTask().executeOnExecutor(
			AsyncTask.THREAD_POOL_EXECUTOR, startId)
    return Service.START_STICKY
}

When the application is now run, it should be possible to touch the Start Service button multiple times. When doing so, the Logcat output should indicate more than one task running simultaneously (subject to CPU core limitations):

I/ServiceExample: Service Running 1
I/ServiceExample: Service Running 2
I/ServiceExample: Service Running 1
I/ServiceExample: Service Running 2
I/ServiceExample: Service Running 1
I/ServiceExample: Service Running 2
I/ServiceExample: Service Running 1
I/ServiceExample: Service Running 2
I/ServiceExample: Service complete 1
I/ServiceExample: Service complete 2

With the service now handling requests outside of the main thread, the application remains responsive to both the user and the Android system.

Summary

This chapter has worked through an example implementation of an Android started service using the IntentService and Service classes. The example also demonstrated the use of asynchronous tasks within a service to avoid making the main thread of the application unresponsive.